1. Field of the Invention
The present invention relates to a secondary radar system particularly for Mode S operation, as is used for air surveillance and required by the ICAO Specification, Annex 10, Part 1, Chapter 3.8.2 and a method for maintaining synchronism within such a secondary radar system.
2. Description of the Prior Art
The construction and operation of secondary radar systems, henceforth called "SSR systems", are well known in the art.
SSR systems consist of ground stations and airborne stations. The ground stations send out interrogation signals via a rotating unidirectional antenna. If these interrogation signals are received by an airborne station, the latter will transmit reply signals after a fixed delay. These reply signals contain, for example, information on the altitude, temperature, and speed of an aircraft equipped with the airborne station, which is also called "transponder". The ground station locates the aircraft by measuring the elapsed time between the transmission of the interrogation signal and the reception of the reply signal and by means of the altitude data contained in the reply signal. Directional information is derived from the position of the rotating antenna. The data-link capability of this system is considerably improved if the transponders on the aircraft are addressed by the ground stations selectively, i.e., if the interrogation signals transmitted by the ground stations are responded to not by all transponders within the range of a ground station, but only by those whose address was contained in the interrogation signal. The addresses are known to the ground stations either from the flight plan or from squitter signals transmitted by the transponders at intervals of 0.8 to 1.2 seconds.
SSR systems as described above suffer from the drawback that information can be exchanged between ground station and airborne station only if the lobe of the rotating antenna sweeps the area in which the respective aircraft and, hence, the respective transponder are located. With increasing air traffic density, this is a constraint that may affect safety.
A solution provided by the present invention for synchronizing the interrogators in the secondary radar system uses a principle that is known for performing time comparisons between satellite ground stations used for communication. (See, for example, the article by D. Kirchner et al. entitled: "A Two-Way Time Transfer Experiment Via ESC-1 Using the Mitrex Modem" appearing in "IEEE Transactions on Instrumentation and Measurement", Vol. 37, No. 3, September 1988, pages 414-417). Also see the article by David Howe in the same IEEE publication, pages 418-423 entitled "High-Accuracy Time Transfer via Geostationary Satellites: Preliminary Results".